Potential Role of Platelet-Activating C-Type Lectin-Like Proteins in Viper Envenomation Induced Thrombotic Microangiopathy Symptom

Novel GPIb-independent platelet aggregation induced by botrocetin: implications for diagnosis and antithrombotic therapy

BACKGROUND

Snake venom botrocetin facilitates von Willebrand factor (VWF) binding to platelet GPIbα and has been widely used for the diagnosis of von Willebrand disease and GPIb-related disorders. Botrocetin is also commonly employed for the development/characterization of antithrombotics targeting the GPIb-VWF axis.

OBJECTIVES

To explore the alternative receptor(s)/mechanisms that participate in botrocetin-induced platelet aggregation.

METHODS

The effects of botrocetin on platelet aggregation were examined using platelets from wild-type, VWF- and fibrinogen-deficient, GPIbα-deficient, IL4Rα/GPIbα-transgenic, ITGA2B and ITGB3-deficient mice, and Bernard-Soulier syndrome and healthy human samples. Platelet-fibrinogen and platelet-VWF interaction were measured using flow cytometry. GPIbα-VWF binding was evaluated utilizing enzyme-linked immunosorbent assay. Botrocetin-αIIbβ3 and botrocetin-GPIbα interactions were measured using enzyme-linked immunosorbent assay and fluorescence anisotropy assays. Heparinized whole blood from healthy donors was examined for thrombus formation and growth in a perfusion chamber.

RESULTS

Botrocetin could induce aggregation of platelets from a Bernard-Soulier syndrome patient and GPIbα-deficient mice as well as platelets lacking the N-terminal extracellular domain of GPIbα. Botrocetin could interact with αIIbβ3 and facilitated αIIbβ3-VWF interaction independent of GPIb. Botrocetin competitively bound to the ligand-binding domain of activated rather than resting αIIbβ3. Although botrocetin-induced platelet aggregation requires VWF, strikingly, in the absence of VWF, botrocetin blocked fibrinogen and other ligand binding to αIIbβ3 and inhibited platelet aggregation and thrombus formation. Consistently, recombinant botrocetin defective in VWF binding inhibited αIIbβ3- and GPIb-mediated platelet aggregation, spreading, and thrombus formation.

CONCLUSION

Our study provides insights into avoiding the misdiagnosis of GPIb-related disorders and developing botrocetin mutants as potential new antithrombotics that may simultaneously target both αIIbβ3 and GPIbα.

A novel snake venom C-type lectin-like protein modulates blood coagulation by targeting von Willebrand factor and coagulation factor IX

Snake venom C-type lectin-like proteins (CLPs) belong to the nonenzymatic proteins. To date, no CLP with both platelet and coagulation factors activating activities has been reported. In this study, a novel CLP, termed protocetin, with molecular weight of 29.986 kDa, was purified from the Protobothrops mucrosquamatus venom (PMV). It consists of α- and β-chains, with 67% similarity in their N-terminal sequence. Protocetin activates glycoprotein Ib (GPIb) by binding to von Willebrand factor (vWF), inducing platelet aggregation. It also activates the intrinsic coagulation pathway by binding to coagulation factor IX. After injection of protocetin into mice at dose of 0.5 µg/g or 1.5 µg/g, it resulted in activation of platelets, a notable reduction in platelet count and prolonged tail bleeding time. Additionally, the plasma activated partial thromboplastin time (APTT) was significantly extended, and the fibrinogen concentration was markedly reduced. Thrombelastogram comfirmed the anticoagulation effect of protocetin. Notably, no microthrombosis was observed in tissues of lung, liver and kidney within 1 h after injection of protocetin into the mice at dose of 0.5 µg/g. This study revealed protocetin as a novel CLP from PMV that has dual functions in activating platelet and coagulation factor IX, thereby modulates coagulation in vivo. This work contributes to a better understanding of the structure and function of snake venom CLP.

Platelet Desialylation Is a Novel Mechanism and Therapeutic Target in Daboia siamensis and Agkistrodon halys Envenomation-Induced Thrombocytopenia

Venom-induced thrombocytopenia (VIT) is one of the most important hemotoxic effects of a snakebite, which is often associated with venom-induced consumptive coagulopathy (VICC). Refractory thrombocytopenia without significant coagulation abnormalities has also been reported after envenomation by some viperid snakes; however, the mechanisms are not well understood and therapeutic strategies are lacking. Here, we found that patients injured by Daboia siamensis or Agkistrodon halys snakes, who were resistant to standard antivenom treatment, had developed coagulopathy-independent thrombocytopenia. Venoms from these viperid snakes, rather than from the elapid snake (Bungarus multicinctus), induced platelet surface expression of neuraminidase-1 (NEU-1), and significantly increased the desialylation of the glycoproteins on human platelets. The desialylated platelets caused by viperid snake venoms were further internalized by macrophages, which resulted in reduced platelet numbers in peripheral blood. Importantly, neuraminidase inhibitor significantly decreased viper venom-induced platelet desialylation, therefore inhibiting platelet phagocytosis by macrophages, and alleviating venom-induced thrombocytopenia. Collectively, these findings support an important role for desialylated platelet clearance in the progression of viper envenomation-induced, coagulopathy-independent thrombocytopenia. Our study demonstrates that the neuraminidase inhibitor may be a potential therapy or adjuvant therapy to treat snakebite-induced thrombocytopenia.

Novel contact-kinin inhibitor sylvestin targets thromboinflammation and ameliorates ischemic stroke

Ischemic stroke is a leading cause of death and disability worldwide. Increasing evidence indicates that ischemic stroke is a thromboinflammatory disease in which the contact-kinin pathway has a central role by activating pro-coagulant and pro-inflammatory processes. The blocking of distinct members of the contact-kinin pathway is a promising strategy to control ischemic stroke. Here, a plasma kallikrein and active FXII (FXIIa) inhibitor (sylvestin, contained 43 amino acids, with a molecular weight of 4790.4 Da) was first identified from forest leeches (Haemadipsa sylvestris). Testing revealed that sylvestin prolonged activated partial thromboplastin time without affecting prothrombin time. Thromboelastography and clot retraction assays further showed that it extended clotting time in whole blood and inhibited clot retraction in platelet-rich plasma. In addition, sylvestin prevented thrombosis in vivo in FeCl3-induced arterial and carrageenan-induced tail thrombosis models. The potential role of sylvestin in ischemic stroke was evaluated by transient and permanent middle cerebral artery occlusion models. Sylvestin administration profoundly protected mice from ischemic stroke by counteracting intracerebral thrombosis and inflammation. Importantly, sylvestin showed no signs of bleeding tendency. The present study identifies sylvestin is a promising contact-kinin pathway inhibitor that can proffer profound protection from ischemic stroke without increased risk of bleeding.

Proteomics and immunocharacterization of Asian mountain pit viper (Ovophis monticola) venom

The venomic profile of Asian mountain pit viper Ovophis monticola is clarified in the present study. Using mass spectrometry-based proteomics, 247 different proteins were identified in crude venom of O. monticola found in Thailand. The most abundant proteins were snake venom metalloproteases (SVMP) (36.8%), snake venom serine proteases (SVSP) (31.1%), and phospholipases A2 (PLA2) (12.1%). Less abundant proteins included L-amino acid oxidase (LAAO) (5.7%), venom nerve growth factor (3.6%), nucleic acid degrading enzymes (3.2%), C-type lectins (CTL) (1.6%), cysteine-rich secretory proteins (CRISP) (1.2%) and disintegrin (1.2%). The immunoreactivity of this viper's venom to a monovalent antivenom against green pit viper Trimeresurus albolabris, or to a polyvalent antivenom against hemotoxic venom was investigated by indirect ELISA and two-dimensional (2D) immunoblotting. Polyvalent antivenom showed substantially greater reactivity levels than monovalent antivenom. A titer for the monovalent antivenom was over 1:1.28x107 dilution while that of polyvalent antivenom was 1:5.12x107. Of a total of 89 spots comprising 173 proteins, 40 spots of predominantly SVMP, SVSP and PLA2 were specific antigens for antivenoms. The 49 unrecognized spots containing 72 proteins were characterized as non-reactive proteins, and included certain types of CTLs and CRISPs. These neglected venom constituents could limit the effectiveness of antivenom-based therapy currently available for victims of pit viper envenomation.

A Strategy for Efficient Preparation of Genus-Specific Diagnostic Antibodies for Snakebites

As said by former United Nations Secretary-General Kofi Annan, "Snakebite is the most important tropical disease you've never heard of." Listed as a priority neglected tropical disease by the World Health Organization, snakebite envenoming (SBE) kills in excess of 125,000 people per year. However, due to the complexity and overlap of snake venom compositions, few reliable venom diagnostic methods for genus-/species-specific identification, which is crucial for successful SBE therapy, are available. Here, we develop a strategy to select and prepare genus-specific snake venom antibodies, which allows rapid and efficient clinical diagnosis of snakebite. Multi-omics approaches are used to choose candidate antigens from snake venoms and identify genus-specific antigenic epitope peptide fragments (GSAEPs) with ideal immunogenicity, specificity, and spatial accessibility. Double-antibody sandwich ELISA kit was established by matching a polyclonal antibody against a natural antigen and a monoclonal antibody that was prepared by natural protein as antigen and can specifically target the GSAEPs. The kit shows the ability to accurately identify venoms from similar genera of Trimeresurus and Protobothrops with a detection limit of 6.25 ng/ml on the snake venoms and a little cross-reaction, thus proving high feasibility and applicability.

Viper venoms drive the macrophages and hepatocytes to sequester and clear platelets: novel mechanism and therapeutic strategy for venom-induced thrombocytopenia

Venomous snakebites cause clinical manifestations that range from local to systemic and are considered a significant global health challenge. Persistent or refractory thrombocytopenia has been frequently reported in snakebite patients, especially in cases caused by viperidae snakes. Viper envenomation-induced thrombocytopenia may persist in the absence of significant consumption coagulopathy even after the antivenom treatment, yet the mechanism remains largely unknown. Our study aims to investigate the mechanism and discover novel therapeutic targets for coagulopathy-independent thrombocytopenia caused by viper envenomation. Here we found that patients bitten by Protobothrops mucrosquamatus and Trimeresurus stejnegeri, rather than Naja. atra may develop antivenom-resistant and coagulopathy-independent thrombocytopenia. Crude venoms and the derived C-type lectin-like proteins from these vipers significantly increased platelet surface expression of neuraminidase and platelet desialylation, therefore led to platelet ingestion by both macrophages and hepatocytes in vitro, and drastically decreased peripheral platelet counts in vivo. Our study is the first to demonstrate that desialylation-mediated platelet clearance is a novel mechanism of viper envenomation-induced refractory thrombocytopenia and C-type lectin-like proteins derived from the viper venoms contribute to snake venom-induced thrombocytopenia. The results of this study suggest the inhibition of platelet desialylation as a novel therapeutic strategy against viper venom-induced refractory thrombocytopenia.

Soy Isoflavones Inhibit Both GPIb-IX Signaling and αIIbβ3 Outside-In Signaling via 14-3-3ζ in Platelet

Soy diet is thought to help prevent cardiovascular diseases in humans. Isoflavone, which is abundant in soybean and other legumes, has been reported to possess antiplatelet activity and potential antithrombotic effect. Our study aims to elucidate the potential target of soy isoflavone in platelet. The anti-thrombosis formation effect of genistein and daidzein was evaluated in ex vivo perfusion chamber model under low (300 s-1) and high (1800 s-1) shear forces. The effect of genistein and daidzein on platelet aggregation and spreading was evaluated with platelets from both wildtype and GPIbα deficient mice. The interaction of these soy isoflavone with 14-3-3ζ was detected by surface plasmon resonance (SPR) and co-immunoprecipitation, and the effect of αIIbβ3-mediated outside-in signaling transduction was evaluated by western blot. We found both genistein and daidzein showed inhibitory effect on thrombosis formation in perfusion chamber, especially under high shear force (1800 s-1). These soy isoflavone interact with 14-3-3ζ and inhibited both GPIb-IX and αIIbβ3-mediated platelet aggregation, integrin-mediated platelet spreading and outside-in signaling transduction. Our findings indicate that 14-3-3ζ is a novel target of genistein and daidzein. 14-3-3ζ, an adaptor protein that regulates both GPIb-IX and αIIbβ3-mediated platelet activation is involved in soy isoflavone mediated platelet inhibition.